Theoretical investigation of the II-VI and IV-VI families of diluted magnetic semiconductors
Description
This dissertation examines the electronic structure and magnetic properties of II-VI and IV-VI dilute magnetic semiconductors (DMS). Properties that are investigated include the exchange energy, magnetic moment, density of states, sources of the magnetic coupling, and the effect that crystal disorder has on the aforementioned parameters. The computational methods employed are the Vienna ab-initio Simulation Package (VASP), and the Layered Korringa-Kohn-Rostoker (LKKR) method. These two methods are based upon density functional theory. VASP relies on the construction of a pseudopotential and a plane wave expansion to model the charge density and wavefunction. LKKR uses multiple scattering theory to find the Green's function and electronic structure. The coherent potential approximation (CPA) can be readily incorporated into the LKKR approach, resulting in a first principle technique that can study a substitutionally disordered random alloy We have studied how the double-exchange, super-exchange, and inter-band exchange are effected by the crystal symmetry of the host, the electronic structure of the transition metal, and geometry of the impurities d-shell. We observed in a few materials that a competition between exchange mechanism is possible. When the sign of the interactions are the same, the result is an unambiguous magnetic ground state. However, when the sign of the competing exchange mechanisms are opposite, the material is expected to have a weaker, often oscillating, magnetic coupling, as a result of magnetic frustration and sensitivity to transition metal spacing and orientation. We have also examined how the chemical interactions may be coupled to the magnetic interactions. This becomes important at high impurity concentrations when the transition metal impurity cannot participate effectively in crystal bonding. In these cases, the transition metal d-orbitals that reside in the gap, and are involved in the exchange, are forced to initiate bonding with the host. This will result in an unexpected magnetic coupling. We note that most models of the transition metal coupling are formulated in the dilute limit The goal of this study was to discover, theoretically, a DMS structure that is both half-metallic and ferromagnetic at room temperature. The Cr doped compounds, and Ni II-VI compounds were found to be the most likely candidates to exhibit these properties. We also seek to establish systematic trends of how the electronic structure and magnetic properties vary as a function of crystal disorder. This is relevant since disorder is always present to some degree in these types of materials as a consequence of the growth techniques used in their fabrication